An anti-trypanosomal factor (ATF) produced by Pseudomonas fluorescens against the Tulahuen strain of Trypanosoma cruzi consisted of two main fractions: ATF-I, exhibited a molecular weight higher than 20,000, was less active against flagellates and produced toxic effects in mice. ATF-II, of molecular weight approximately 1,000, was quite active against the parasites, and well tolerated in mice. In view of the protective effects observed in vivo and of the potential significance of these findings on the chemotherapy of Chagas' disease further studies on the chemical characterization of the ATF and on its pharmacological and toxicological properties will be made.